The present invention relates to an automatic transmission, and more particularly relates to an automatic transmission in which a means is provided for reducing shift shock upon upshifting.
In an automatic transmission in which, in order to upshift from a certain lower speed stage to a speed stage one higher, a lower side friction engagement device is disengaged substantially simultaneously as a higher side friction engagement device is engaged, providing good mutual timing for this engagement and disengagement is very important. If the lower side friction engagement device is disengaged too early relative to the engagement of the higher side friction engagement device, then the rotational speed of the engine coupled to the transmission increases suddenly, and then as the higher side friction engagement device is subsequently engaged a troublesome shifting shock can be caused, and also the friction engagement devices and other parts of the transmission can be unduly deteriorated. On the other hand, if the lower side friction engagement device is disengaged too late relative to the engagement of the higher side friction engagement device, then the consequent overlapping of the engagement of the two friction engagement devices causes the transmission to be substantially engaged in two different speed stages at the same time; and this may cause at least temporary locking up of the transmission, leading to unpleasant shifting shock, and also can severely damage the members of the transmission such as the friction engagement devices.
In order to control this mutual timing of the simultaneous disengagement and engagement of the lower and higher side friction engagement devices, in Japanese Patent Publications Nos. 48-20655, 48-21368, and 49-40585 there has been proposed the concept of variably controlling the flow resistance of the drain path which is provided from the lower side friction engagement device according to the actuating fluid pressure that is being supplied to the higher side friction engagement device. In such a construction, when the actuating pressure of the higher side friction engagement device rises above a certain level, so that said higher side friction engagement device is engaged to a certain degree, the lower side friction engagement device is quickly drained so as to disengage it. However, although this concept is helpful, upshifting shock still occurs in the case that the upshifting is being made with the throttle opening relatively low, as for example if a shift-up occurs when the vehicle has been moved off from rest suddenly and then the accelerator pedal has been substantially released. This is because, if the throttle opening is high, much slipping of the friction engagement devices is caused by high engine torque during upshifting, and so the aforementioned effective establishment of two different speed stages at the same time during the transitory period of overlapping of the engagement of the two friction engagement devices is not so noticeable; but this masking effect of slippage is not available when the engine torque is low. In extreme cases at low throttle opening, the shock may be so great as to give the impression that the braking system of the vehicle has been suddenly applied, and needless to say this can cause severe transmission damage over a period of time.